Verified answer

you need to forward your ports.

go to www.portforward.com for help specific to your router.

if you know how to forward ports then the ports you need to forward are

Port 88 (UDP)

Port 3074 (UDP and TCP)

Port 53 (UDP and TCP)

Port 80 (TCP)

Edit: those are the xbox ports

ps3 is tcp: 80, 443, 5223

udp: 3478, 3479, 3658

Since I don't know what system you are using, here is a link to a website with videos on how to do it for all three! Enjoy!! XD

http://www.joinaclan.com/2011/11/cod-mw3-modern-wa...

Your "Network Adress Translations" is what your router lets into you home through the internet. It's different for every router, but what you should do is look for a guide to port forward all of the ports needed for the xbox, and then you will have no problems connecting to anyone (as long as their settings aren't strict)